This application relates to vibration reduction in a GM displacer/regenerator, and more particularly, relates to vibration reduction in a pneumatically-driven GM displacer/regenerator. Cryogenic refrigerators of the GM type frequently include a multi-stage displacer/regenerator as a key element in expanding high pressure gaseous refrigerant to achieve extremely low temperatures.
There is an abundance of prior art that describes various pneumatically-driven and mechanically-driven displacers and their operations in cryogenic systems and in achieving cryogenic temperatures. For example, basic principals of operation are described in the original Gifford-McMahon (GM) U.S. Pat. No. 2,906,101, issued Sep. 29, 1959. In that patent, which is incorporated herein by reference, the displacer is reciprocatingly driven in a cylinder by a conventional crank mechanism. Thus, low temperature refrigeration is effected with auxiliary equipment, such as connecting rods, crank shafts, or the like, to cycle the displacer. These mechanical parts produce mechanical vibrations that in many instances are undesirable and shorten the time between necessary maintenance or repairs.
U.S. Pat. No. 3,620,029, issued Nov. 16, 1971 by the present inventor, and incorporated herein by reference, replaces mechanical drive of the displacer with a pneumatic drive. The mechanical problems associated with the crank type drive, or cam type drive, as in other designs, are substantially eliminated and the operating life of the systems has been enhanced by such pneumatic drives. However, other mechanical problems, noise and vibration producing problems arise through the use of the pneumatically-driven displacer. These problems have roots also in the thermodynamics of the refrigeration cycle.
In a mechanically-driven or pneumatically-driven displacer/expander, the displacer includes a piston that reciprocates within a cylinder. When the piston moves to what is known as the "bottom" of the cylinder, it is most desirable thermodynamically that the clearance volume be zero, or as near to that volume as possible. Thus, unless careful control is provided for the motion of the displacer, collisions can occur between the displacer piston and the closed end of the cylinder. These collisions create noise and vibration. Also, when the displacer moves in the opposite direction, unless careful control is provided, there can be an impact when the displacer is at the "top" of its stroke. Further noise and vibration are produced. (The use of the words "top", "bottom", "up", "down", and the like does not necessarily indicate a physical orientation. No orientation is excluded from use.)
The original GM U.S. Pat. No. 2,906,101, describes a rectangular pressure-volume (P-V) diagram but actually it is best from a thermodynamic standpoint to close the inlet valve before the displacer reaches the top. This causes the gas pressure in the expander to drop before the displacer reaches the top. Similarly it is best to close the exhaust valve before the displacer reaches the bottom. This causes an increase in pressure before the displacer reaches the bottom. In a pneumatically driven expander this causes the displacer to decelerate before it reaches the end of the stroke.
Many vibration isolation systems have been developed to improve cycle efficiency and to prevent collisions between the displacer and its surroundings, or where collisions occur, to reduce vibrations caused by the impact. These include both electrical and mechanical concepts.
For example, repelling magnets have been used to constrain the motion of the displacer at the top and bottom ends of its motion. Elastomer vibration absorbers have been used with some success. However, these devices are only effective at the warm end of the displacer motion, but are not able to operate effectively at the cryogenic temperatures. Therefore, impact forces at the cold end have been absorbed, for example, using delrin plastic pads, which can take the low temperatures. However, there is still a considerable impact and vibration problem when using delrin absorbers. Such impacts and vibrations have been known to affect the quality and resolution of images obtained in MRI apparatuses that use cryogenically cooled magnets.
What is needed is an improved expander that has the advantages of a simplified pneumatic drive, long operating life, low vibration in operation and an efficient thermodynamic cycle.